5i'
ANNEX D
(to Recommendation Q.921)
State transition table of the point-to-point procedures
of the data link layer
D.1 The state transition table presented in Tables D-1/Q.921
to D-3/Q.921 is based on the eight basic states (see S B.2) recog-
nized in the SDL representation and the related transmitter and
receiver conditions.
The state transition table relinquishes to any partitioning of
the procedures. It is conceptual and does not prevent a designer
from partitioning in his implementation. Moreover, all the
processes related to primitive procedures, the management of queues
and the exchange of information between adjacent layers are concep-
tual, not visible from outside of the system and would not impose
any constraints on the implementation.
The eight basic states apply to both the transmitter and the
receiver within one data link layer entity. However, some of the
conditions are confined to the transmitter (e.g. "peer receiver
busy"), whilst some are confined to the receiver (e.g. "REJ
recovery"). This implies, if the concept of non-partitioning is
adopted, that each transmitter condition has to be combined with
each receiver condition resulting in composite states. This state
transition table comprises 24 composite states representing the
8 basic states and the related combinations of transmitter and
receiver conditions.
Events are defined as follows:
a) primitives;
b) repertoire of frames to be received;
- unnumbered frames (SABME, DISC, UA, DM, UI, FRMR)
- supervisory frames (RR, REJ, RNR)
- information frame (I);
c) internal events (servicing of queues, expiry of
timers, receiver busy condition).
The actions to be taken when an event occurs whilst in a
specific state comprise:
i) transition to another state
ii) peer-to-peer frame to be transmitted
iii) primitives to be issued
iv) timer actions
v) retry counters
vi) state variables
vii) P/F bit setting
viii) discarding contents of queues.
D.2 Key to the state transition table
D.2.1 Definition of a cell of the state transition table
Diagram [T17.921], p.
D.2.2 Key to the contents of a cell
| Impossible by the definition of the data link
layer service.
/ Impossible by the definition of the peer-to-peer
data link procedures.
- No action, no state change.
V(S) = V(A) = N(R) Collective term for the two
actions V(S) = N(R) and V(A) = N(R).
Timer T200 Start timer T200 if not already running.
TX ACK The acknowledgement of the received I frame
may be conveyed by an I frame associated with the information flow
in the opposite direction or a supervisory response frame, as
appropriate.
"DISCARD" Indicates the discarding of the informa-
tion contained in the information field of the I frame.
(A-O) The codes used in MDL-ERROR-INDICATION sig-
nals are defined in Table II-1/Q.921 in Appendix II. When multiple
codes are shown, only one applies.
Diagram [T18.921] + Note, p.
blanc
H.T. [1T19.921]
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TABLE D-1/Q.921 (1 of 10)
{
State transition table: receiving primitive
}
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BASIC STATE TEI UNASSIGNED ASSIGN AWAITING TEI ESTABLISH AWAITING TEI TEI ASSIGNED AWAITING ESTABLISHMENT AWAITING RELEASE
RECEIVER CONDITION See Note
STATE NUMBER 1 2 3 4 5.0 5.1 5.2 6
DL-ESTABLISH-REQUEST {
MDL-ASS-IDN
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3
} | | | | | | 3 I {
RC=0
TX SABME P=1
START T200
5.0
} I {
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DISC. I QUEUE
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5.0
} I I
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DL-RELEASE-REQUEST I I I DL-REL-CONF I | | 5.2 I I
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DL-DATA-REQUEST I I I I I DATA INTO I QUEUE I I
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I FRAME IN QUEUE V(S)